MRI and PET have demonstrated their strengths in the diagnosis of breast cancer. In this proposal we bring the 2 modalities together and build on these strengths. The goal of this proposal is to develop a new breast imaging diagnostic instrument composed of magnetic resonance imaging (MRI) system and novel positron emission tomography (PET) scanner. The PET system will be placed inside the MRI magnetic field next to a breast magnetic coil. The integrated MRI-PET system will provide: MR-(anatomic/nuclear molecular image), Nuclear (nuclear medicine based molecular image) and Combined (fused image) images without the necessity of moving the patient. Use of the combined modalities will allow obtaining very high sensitivity and specificity of diagnosed lesions during the same exam. The technological novelty of the proposed approach lays in the development of a novel PET system and new reconstruction software. The PET system will be built in a form of inexpensive small size ring scanner, which is not sensitive to magnetic fields present in the MRI systems. In the Phase I effort we will develop two PET scanner modules and processing electronics and reconstruction software. The two PET modules will be tested for operation in realistic conditions using MRI clinical system at UCLA. PROPOSED COMMERCIAL APPLICATIONS: In this proposal, we will build a small PET system for imaging the breast that can be used in the magnetic field of the MRI scanner. The market for these systems is expected to be in the 100's of millions of dollars. In addition, the combination MR-PET system could reduce the number of biopsies by giving unequivocal diagnosis for breast cancer in many cases that require biopsy today. The cost of one biopsy is approximately $3,000. Approximately 2 million biopsies are performed every year. This system could save $100's of millions of dollars annually.